Table of contents

1. A Review of General Chemistry5h 5m
2. Molecular Representations1h 14m
3. Acids and Bases2h 46m
4. Alkanes and Cycloalkanes4h 17m
5. Chirality3h 39m
6. Thermodynamics and Kinetics1h 22m
7. Substitution Reactions1h 48m
8. Elimination Reactions2h 30m
9. Alkenes and Alkynes2h 9m
10. Addition Reactions3h 19m
11. Radical Reactions1h 58m
12. Alcohols, Ethers, Epoxides and Thiols2h 42m
13. Alcohols and Carbonyl Compounds2h 17m
14. Synthetic Techniques1h 26m
15. Analytical Techniques:IR, NMR, Mass Spect7h 3m
16. Conjugated Systems6h 13m
17. Ultraviolet Spectroscopy51m
18. Aromaticity2h 34m
19. Reactions of Aromatics: EAS and Beyond5h 1m
20. Phenols55m
21. Aldehydes and Ketones: Nucleophilic Addition4h 56m
22. Carboxylic Acid Derivatives: NAS2h 51m
23. The Chemistry of Thioesters, Phophate Ester and Phosphate Anhydrides1h 10m
24. Enolate Chemistry: Reactions at the Alpha-Carbon1h 53m
25. Condensation Chemistry2h 9m
26. Amines1h 43m
27. Heterocycles2h 0m
28. Carbohydrates5h 53m
29. Amino Acids4h 20m
30. Peptides and Proteins2h 42m
31. Catalysis in Organic Reactions1h 30m
32. Lipids 2h 50m
33. The Organic Chemistry of Metabolic Pathways2h 52m
34. Nucleic Acids1h 32m
35. Transition Metals6h 14m
36. Synthetic Polymers1h 49m

10. Addition Reactions

Alkyne Halogenation

Organic Chemistry

10. Addition Reactions

Alkyne Halogenation

Previous problemNext problem

1:53 minutes

Problem 52

Textbook Question

Trans addition is heavily favored for the addition of Br₂ and Cl₂ to alkynes. With chlorination, however, more of the syn addition product is formed. Rationalize this fact in light of your answer to Assessments 10.50 and 10.51.

Verified step by step guidance

Understand the mechanism of halogen addition to alkynes: When Br₂ or Cl₂ is added to an alkyne, the reaction typically proceeds through a halonium ion intermediate. This intermediate is a three-membered ring structure where the halogen is bonded to both carbons of the alkyne, creating a strained cyclic structure.

Analyze the stereochemistry of the addition: In the case of Br₂, the halonium ion intermediate is more stable due to the larger size and polarizability of bromine. This stability favors a trans addition mechanism, where the nucleophilic attack by the second halide ion occurs on the opposite side of the halonium ion, leading to anti stereochemistry.

Consider the differences with Cl₂: Chlorine is smaller and less polarizable than bromine, which makes the halonium ion intermediate less stable. This reduced stability allows for a competing pathway where the addition occurs via a concerted mechanism, leading to some syn addition products.

Relate this to the experimental observation: The higher proportion of syn addition products in chlorination can be rationalized by the fact that the less stable chloronium ion intermediate allows for alternative reaction pathways, such as direct concerted addition, which does not involve the formation of a halonium ion.

Conclude the reasoning: The difference in the stereochemical outcome between bromination and chlorination is due to the relative stability of the halonium ion intermediates. Bromine's larger size and polarizability favor trans addition, while chlorine's smaller size and lower polarizability allow for more syn addition products to form.

Verified video answer for a similar problem:

This video solution was recommended by our tutors as helpful for the problem above

Video duration:

Play a video:

Was this helpful?

Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Alkyne Reactivity

Alkynes are unsaturated hydrocarbons containing at least one carbon-carbon triple bond. Their reactivity is significantly influenced by the nature of the substituents and the type of addition reactions they undergo. In the case of halogen addition, alkynes can react with diatomic halogens like Br₂ and Cl₂, leading to the formation of vicinal dihalides through either syn or anti addition mechanisms.

Syn vs. Anti Addition

In organic chemistry, syn addition refers to the addition of two substituents to the same side of a double or triple bond, while anti addition involves adding them to opposite sides. The stereochemistry of the addition can affect the stability and formation of products. For alkynes, syn addition is often favored under certain conditions, particularly with chlorination, due to steric and electronic factors that stabilize the transition state.

Halogenation Mechanism

The halogenation of alkynes typically proceeds through a cyclic halonium ion intermediate, which can lead to different stereochemical outcomes depending on the conditions. For bromination and chlorination, the formation of the halonium ion allows for both syn and anti addition pathways. The preference for syn addition in chlorination can be attributed to the lower steric hindrance and the ability of the halogen to stabilize the transition state more effectively than in bromination.

Watch next

Master Double halogenation of alkynes. with a bite sized video explanation from Johnny

Start learning

Related Videos

Related Practice

Textbook Question

What is the major product of the reaction of 1 mol of propyne with each of the following reagents?

c. Br₂ (1 mol)/CH₂Cl₂

Textbook Question

In the addition of just 1 mole of bromine to 1 mole of hex-1-yne, should the hex-1-yne be added to a bromine solution or should the bromine be added to the hex-1-yne? Explain your answer.

Textbook Question

For the alkynes shows here, show the product(s) expected to form when treated under the following conditions: (viii) Cl₂ (1 equiv.). If you expect two products, show both.

(c)

Textbook Question

For the alkynes shows here, show the product(s) expected to form when treated under the following conditions: (ix) Br₂ (2 equiv.). If you expect two products, show both.

(c)

Textbook Question

Drawing on what you know about the stereochemistry of alkene addition reactions,

a. write the mechanism for the reaction of 2-butyne with one equivalent of Br₂.

Textbook Question

Drawing on what you know about the stereochemistry of alkene addition reactions,

b. predict the configuration of the product of the reaction.

Textbook Question

Show how hex-1-yne might be converted to

d. 1,1,2,2-tetrabromohexane.

Textbook Question

Predict the product(s) that would result when the alkenes shown here are allowed to react under the following conditions: (v) 1. Hg(OAc)₂ , H₂O 2. NaBH₄

(b)

Show more practices

Download the Mobile app

Privacy

Do not sell my personal information

Accessibility

Patent notice

Sitemap